I've not heard or read anything regarding organic material being classified as a mineraloid, but that can change or has changed(?). Clays, such as illite, chlorite, etc. are minerals, but "clay" is also a particle size. So clay minerals, as well as clay-sized particles (of other minerals), comprise shale. (Just an FYI for the readers.

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Iron, the metal, may have a higher melting point than magnesium metal, but Mg-rich forsterite is a far more stable mineral under higher temps and pressures (as present in the mantle) than the Fe-rich fayalite. Therefore, forsterite will crystallize first from higher temp ultramafic/mafic melts, rather than fayalite.